U.S. Patent Publication No. 2012/0034150 A1, published Feb. 9, 2012, and titled “Method for Producing Solid Carbon by Reducing Carbon Oxides” discloses background information hereto. The subject matter of this application also relates to the subject matter of U.S. Patent Publication 2014/0021827 A1, published Jan. 23, 2014, titled “Primary Voltaic Sources Including Nanofiber Schottky Barrier Arrays and Methods of Forming Same;” and the subject matter of International Patent Publication WO 2014/011631, published Jan. 16, 2014, titled “Solid Carbon Products Comprising Carbon Nanotubes and Methods of Forming Same.” The disclosures of each of these references are hereby incorporated herein in their entirety by this reference.
Powder metallurgy is the process of making a solid object out of one or more powdered materials. Specific techniques for working with various materials include manufacturing the powder, blending the powder to get the desired bulk composition, compressing the powder into a solid object, and in some cases sintering the solid object to further consolidate and change the properties of the solid object. Powder metallurgy is used on a wide variety of materials.
Solid carbon has a wide variety of allotropes and morphologies. Of particular interest are the nanostructured carbons, which include buckminsterfullerenes, carbon nanotubes (e.g., single-wall carbon nanotubes, multi-walled carbon nanotubes), nanodiamonds, graphene, nanofibers, amorphous carbons (both sp2- and sp3-bonded) and others known to the art. Many nanostructured carbon materials are extremely valuable, because of their unique physical and chemical properties. For example, various nanostructured materials are among the strongest, most thermally conductive, electrically conductive, corrosion resistant, abrasion resistant, and/or heat resistant, materials known. When used as additives, even small amounts of nanostructured carbon can significantly improve the performance of materials. Because of their high cost of production, however, nanostructured carbon is generally compounded as an additive with other substances.
Electrical conductors, such as wires and electrodes, are ubiquitous as components of various consumer products, industrial equipment, etc. Electrical conductors are commonly formed of metals, such as copper, aluminum, gold, platinum, steel, etc. The electrical conductivity of a material measures the material's ability to conduct an electrical current. Materials having relatively higher electrical conductivities may be well suited to carrying large currents, because such high-conductivity materials are less susceptible to heating than materials having relatively lower electrical conductivities.
Design of industrial and consumer products often requires balancing a number of considerations, including electrical properties, physical properties, material costs, and manufacturing concerns. In the semiconductor industry, components are typically designed to be compact, reliable, and energy-efficient, with low manufacturing costs. Electrical conductors in such applications may be formed of low-cost materials having high electrical conductivity and adequate physical strength. Industrial equipment, on the other hand, may be designed with different objectives; for example, compactness may be less important than reliability.
Electrodes in electric arc furnaces endure harsh chemical, mechanical, thermal and electrical conditions. Many methods have been employed to develop electrode materials that are suitable for these service conditions. Electrodes for electric arc furnaces are currently formed as artificial graphite in a multi-step process that includes mixing a high purity carbon (pet coke, needle coke, etc.) with a hydrocarbon binder (e.g., a tar), extruding the electrode as a green form, pyrolizing the green form to pyrolize the tar and hydrocarbons in the electrode, and then sintering the electrode. The steps are energy- and time-intensive.
Carbon nanotubes are known to have high electrical conductivity. Carbon nanotube aggregates are described in U.S. Pat. No. 7,740,825, issued Jun. 22, 2010, and titled “Method of Forming a Carbon Nanotube Aggregate.” Unpurified multilayer carbon nanotubes are fired and oxidized, then washed to remove amorphous carbon and catalytic metal. The washing process uses acid, base, and water in sequence. The purified carbon nanotubes are treated with fluorine to form C—F bonds in the carbon nanotubes. The fluorinated carbon nanotubes are pressed, sintered, and deaerated to bind the carbon nanotubes together and to remove the fluorine. Chemical bonding occurs between at least some of the carbon nanotubes at their contact points with adjacent carbon nanotubes. No binder or resin is used.
Carbon nanotubes are known to form solid masses, without fluorination or sintering as described in U.S. Pat. No. 6,899,945, issued May 31, 2005, and titled “Entangled single-wall carbon nanotube solid material and methods for making same;” and in U.S. Pat. No. 7,288,238, issued Oct. 30, 2007, and titled “Single-wall carbon nanotube alewives, process for making, and compositions thereof” It would be desirable to be able to form solid masses with high mechanical integrity suitable for applications such as electrodes for electric arc furnaces.
It would be beneficial to provide solid carbon objects with the properties of nanostructured carbon materials, particularly solid carbon electrical conductors with improved electrical and physical properties, that are relatively inexpensive, and that may be easily manufactured.